Dimethylnaphthalenes ("DMN") are valuable for use in a variety of chemical manufacturing processes. 2,6-dimethylnaphthalene ("2,6-DMN"), e.g., is particularly valuable. 2,6-DMN is a precursor for the manufacture of 2,6-naphthalene dicarboxylic acid ("2,6-NDA") and 2,6-naphthalene dicarboxylate ("2,6-NDC"). 2,6-NDA and 2,6-NDC are monomers either of which, when combined with ethylene glycol, reacts to make polyethylene naphthalate ("PEN"), a polyester which provides superior strength and heat resistance compared to polyethylene terephthalate ("PET") in applications such as films (e.g., photographic film), fibers, containers and molded parts and has, therefore, unique commercial advantages. Thus, there has been recently worldwide increasing interest in the production of 2,6-DMN, 2,6-NDA and 2,6-NDC.
Naturally, there is an interest in having an economical and efficient process for their manufacture. 2,6-NDA and 2,6-NDC production is similar to that of terephthalic acid in that a 2,6-dialkylnaphthalene, e.g., 2,6-DMN, is oxidized to the corresponding acid by using well established methods. Thus, the challenge lies in the manufacture of dialkylnaphthalenes (especially DMN's) from which the 2,6-dialkylnaphthalenes are produced then via a variety of methods including isornerization and separations. As a result, it is highly desirable to find an economical way to manufacture DMN's, particularly 2,6-dialkylnaphthalenes, and especially 2,6-DMN.
Several conventional methods for producing 2,6-naphthalene dicarboxylic acid precursors are known. These methods, which all lead to production of DMN's, can be grouped as follows: (1) alkenylation/cyclization/dehydrogenation, starting with a monocyclic aromatic and a diolefin, (2) reforming/recovery from kerosene fractions, (3) recovery from cycle oil produced in fluid catalytic cracking (FCC) operations, and (4) transalkylation of naphthalene with polyalkylbenzenes. In addition, alkylation (e.g., methylation or propylation) of naphthalene is also an alternative method to produce dialkylnaphthalenes.
In regards to the first category of methods listed above, U.S. Pat. No. 4,990,717 discloses a method for production of a monoalkenylated aromatic via base-catalyzed reactions. The reactions are performed by reacting an alkylbenzene (e.g., ortho-xylene or para-xylene) with a C.sub.4 or C.sub.5 conjugated diene over a fixed bed of supported alkali metal catalyst. Similar reactions using a dispersed alkali metal catalyst are also disclosed in a number of even earlier patents such as U.S. Pat. Nos. 3,953,535, 3,954,895 and 3,954,896. Another known process teaches a method for producing pentyltoluenes. This method is disclosed in U.S. Pat. No. 3,931,348 (the '348patent). The '348 patent teaches producing DMN's from a pentyltoluene/pentenyltoluene obtained from addition of p-xylene and butene or butadiene.
These various approaches are each aimed at making pentyltoluenes/pentenyltoluenes which can be then selectively converted. The conversion is via a subsequent cyclization and dehydrogenation step to DMN's which belong to the same DMN triad as 2,6-DMN (vide infra for the DMN triads). Discovery of an alternative method to these known methods for making DMN's is highly desirable. These known methods are expensive because they involve the relatively expensive starting materials, the relatively expensive base catalysts and the restrictions on manufacturing those DMN precursors which can be selectively converted to the DMN's belonging to the 2,6-DMN triad.
Other methods for the two-stage process of cyclization and dehydrogenation reactions are disclosed, for example, in U.S. Pat. Nos. 5,012,024, 5,030,781, 5,073,670 and 5,118,892. In those references, cyclization of monoalkenylated toluene is accomplished over a zeolite catalyst containing platinum and copper to a dimethyltetralin such as 1,5-dimethyltetralin. These references teach that dehydrogenation of dimethyltetralins typically takes place over a platinum/rhenium catalyst supported on alumina. This method is also undesirable since it involves at least two distinct steps and is relatively expensive.
Yet another known process teaches alkylating toluene with 1-pentene. This method is taught in U.S. Pat. No. 5,043,501. The resulting alkylate is then catalytically dehydrocyclized by contacting with a zeolite L to produce DMN's. This process, however, has many drawbacks. Pure 1-pentene streams are unduly expensive and the yield of DMN's with zeolite L is very poor.
A combined cyclization and dehydrogenation operation is taught in U.S. Pat. No. 5,068,480. The process comprises subjecting 2-methyl-1-(p-tolyl)-butene, 2-methyl-1-(p-tolyl)-butane or a mixture thereof to cyclization and dehydrogenation in the presence of a catalyst comprising lead oxide and/or indium oxide and aluminum oxide. However, similar to the known methods described before, this method intends to make DMN's which belong to the same CMN triad as 2,6-DMN.
As regards the second method stated above for making DMN's, several patents (e.g., U.S. Pat. No. 4,963,248; Japanese Patent Nos. 02,247,136, 02,247,137, 02,304,034 and 03,038,532) describe the reforming of hydrotreated kerosene fractions and recovery of DMN's from the reformate. These references claim improved results are obtained by first removing the normal paraffins from the kerosene fraction using molecular sieve separations. Reforming is typically done over a platinum on alumina catalyst at 750-1020.degree. F. and about 350 psig in the presence of hydrogen to obtain a reformate containing about 18 percent DMN's. 2,6-DMN can be separated from the reformate via distillation, molecular sieve adsorption, crystallization or complexation. This method does not meet the current requirements for economy since the process results in a low DMN yield and even lower 2,6-DMN yield. In addition, the resulting DMN mixture usually consists of all 10 DMN isomers, which is typical of the DMN products directly manufactured from refinery streams and results in inefficiency in the subsequent steps of producing 2,6-DMN via isomerization and separations.
As regards the third method stated above for making DMN's, several references teach recovery of 2,6-DMN from FCC Light Cycle Oil (proposed in a study by Eldib Engineering Co., see Chemical Week, Jun. 24, 1992, p. 27; European Chemical News, Sep. 28, 1992, p. 30; Chemical Marketing Reporter, Oct. 12, 1992; Chemical Week, Nov. 14, 1992, p. 39). As disclosed, the material, which is ordinarily used as a diesel fuel blendstock, contains about 1.75% 2,6-DMN. It is disadvantageously accompanied by all other 9 DMN isomers as in the case of kerosene described above. A key problem with this approach is that the FCC Cycle Oil contains high concentrations of sulfur and nitrogen compounds. These contaminants poison zeolites used in recovery of 2,6-DMN and also isomerization catalysts. To remove the sulfur and nitrogen compounds and to reduce them to about 10 ppm each, hydrotreating at severe conditions is required. Hydrotreating under such severe conditions inevitably causes hydrogenation and hydrocracking of DMN's, resulting in a great reduction in the yield of DMN's.
As regards the fourth method stated above for making DMN's, several patents teach methods of manufacturing DMN's by transalkylation of naphthalene or 2-methyinaphthalene. These patents are, for example, U.K. Patent No. 2,246,788A and European Patent No. 0,494,315 A1. However, it is apparent that such methods are always associated with by-products such as tricilkylnaphthalenes and have a low efficiency. These by-products are undesirable due to reduced yield of the desired products and added complexity and cost to the process because of the need for further separation steps. Similar drawbacks are also unavoidable with the methods of making DMN's by alkylation of naphthalene or methyinaphthalenes.
Accordingly, there exists a need in the petroleum industry for a lower-cost method than presently exists of producing DMN's. The method of this invention provides such a method.